Hard Knott is a hill in the English Lake District, at the head of Eskdale. The Hardknott Pass passes between this hill and Harter Fell to the south, joining Eskdale with the Duddon Valley. There is a steep single-track motor road over the pass with gradients of up to 30% (1 in 3), reaching a maximum height of 393 metres.

Hardknott Castle

On the western side of the pass, on a rocky spur giving a superb view (originally from a miltary point perspective, but equally true for the casual visitor nowadays) over the River Esk, are the remains of a Roman fort. The fort has been commonly known in recent times as Hardknott Castle, but identified from the Ravenna Cosmography as the Mediobogdo fort, situated along the road between the forts of Galava (Ambleside) and Glannoventa (Ravenglass). The Roman garrison here was a detachment of 500 cavalry from the Dalmatian coast.

Built between about 120 and 138 AD, the low walls of the fort were "restored" some years ago, the slate course showing the height of the walls before their rebuilding. Outside the square of the fort is the remains of the bath house, and the levelled parade ground, which is considered to be the finest surviving example.

The fort is square with rounded corners, 114 metres long externally, or 105 metres internally, the rampart wall being about 1.7 metres thick with ditches adding to the total width of the rampart.

The fort is on land owned by the National Trust, and maintained by English Heritage.